1. Field of the Invention:
This invention relates to a photometric device for a camera of the kind measuring light by dividing a photographing field into a plurality of areas.
2. Description of the Related Art:
The conventional photometric device of the above stated kind which measures light by arranging an image forming lens and a light receiving photosensitive element in the rear of the exit face of a pentagonal prism has an advantage in that the simple structural arrangement of the device readily permits finding a space within the camera for arranging the photometric system thereof without difficulty. However, since the image forming lens (light receiving lens) must be disposed in a position away from the optical axis of a view finder, it is a shortcoming of the device that the device is inferior in the so-called F-number characteristic which indicates a proportional relation between the measured light quantity and the F-number of a photo taking lens. The reason for this shortcoming is as follows:
FIG. 4 of the accompanying drawings shows the optical system arrangement of a single-lens reflex camera having a photometric device of the above stated kind. The illustration includes a focusing screen 1; a pentagonal prism 2; an image forming lens 3; a light receiving element 4; an eyepiece 5; a photo taking lens 6; and a quick return mirror 7. FIG. 5 is a development view showing the photometric optical path and a view-finder sighting optical path. The illustration of FIG. 5 includes a view finder optical axis O; a view finder sighting optical path K; and a photometric optical path M located in the middle part of the focusing screen 1. Other reference numerals denote the same things as in FIG. 4. As apparent from FIG. 5, the image forming lens 3 and the light receiving element 4 must be arranged in an eccentric position away from the view finder optical axis O for the purpose of saving the sighting optical path K of the view finder from being shaded. As a result, as indicated by a hatched part, light from the photo taking lens 6 obtained at a dark F-number fails to reach the light receiving element 4. Therefore, there arises a problem that a measured light quantity obtained at a dark F-number becomes not proportional to the F-number of the photo taking lens 6.
The relation of the measured light quantity to the F-number is as shown in FIG. 6, wherein: The F-numbers of the photo taking lens 6 are shown on the axis of abscissa. On the axis of the ordinate are shown a number of steps E of the measured light quantity which is arranged on the basis of F/1.4 of the measured light quantity and, which can be expressed as E=log.sub.2 Q1-log.sub.2 Q0, wherein Q0 represents the F/1.4 of the measured light quantity and Q1 the measured light quantity obtained at each F number. A line I denotes an ideal F-number proportionality. A broken line curve R denotes the above stated F-number proportionality of the conventional photometric device. As apparent from FIG. 6, in the case of the conventional photometric device, the measured light quantity ceases to be proportional to the F-number for dark F-numbers. Besides, the F-number proportionality of the conventional device is inadequate also for brighter F-numbers, because: The photometric optical system of the conventional device cannot be allowed to have a sufficiently large aperture. Therefore, light at bright F-numbers is not allowed to be completely incident on the the light receiving element 4. In the case of a photometric device of the type arranged to measure the light of the whole field by means of a single sensor, this problem is generally solved by detecting the full-open F-number of the photo taking lens 6 with a signal pin or the like and by correcting the measured light quantity accordingly.
While the conventional photometric device of this kind has the above stated intrinsic problem, the seriousness of the problem is mitigated to a considerable degree in actuality by the diffusing action of the matted surface D of the focusing screen 1. This diffusing action which is as shown in FIG. 7 allows the light obtained at a dark F-number to be incident via the image forming lens 3 upon the light receiving element 4. However, if the photometric device is arranged to have the light receiving element 4 divided into many parts and to measure the light of each of a plurality of areas of the field discretely from other areas, accurate light measurement is hardly possible by mere correction of the F-number.
In case that the light receiving face of the above stated light receiving element 4 is divided into a plurality of areas as shown in FIG. 8 including a center area 4A, a middle or intermediate area 4B which adjoins the center area 4A in a concentric manner and four peripheral areas 4C1, 4C2, 4C3 and 4C4, light measurement outputs obtained with a lens of short focal length and with a lens of long focal length lens at the same F-number differ from each other despite the F-number correction. In the optical arrangement as shown in FIGS. 4 and 5, the light measurement outputs vary to a great degree particularly for the lower part of the photographing image plane, or for the upper part of the image plane in case the light receiving element 4 is arranged as shown in FIG. 9. The upper or lower positional relation obtained on the light receiving face is shown in FIG. 8 as viewed through the view finder. In the case of the arrangement of the light receiving element 4 shown in FIG. 9, the peripheral areas 4C3 and 4C4 are located in the upper part and the peripheral areas 4C1 and 4C2 in the lower part.
FIG. 9 shows the reason why the above stated difference arises. A Fresnel lens is provided on the focusing screen 1 of the single-lens reflex camera for the purpose of imaging the exit pupil on the eye to permit sighting the whole view finder field with sufficient light. This Fresnel lens enables the pupil of the image forming lens 3 to be projected within the area of the exit pupil 3a of the photo taking lens 6 as shown in FIG. 9. Each of the areas 4A to 4C are arranged to measure the light of an area around a corresponding one of the areas 1C1 (1C2), 1A and 1C3 (1C4) of the focusing screen 1 through the image forming lens 3. However, the light flux incident on each of these areas of the element 4 is caused to have light come from the area of the exit pupil 3a of the photo taking lens 6. Some of diffused rays of light from areas other than that of the exit pupil 3a are of course caused to come to each area of the element 4 due to the diffusing property of the focusing screen 1. However, the incident light directly from the area of the exit pupil 3a is received in a dominantly effective amount as measured light quantity. FIG. 9 shows the exit pupil position 6p of the photo taking lens 6 on the assumption that the Fresnel lens of the focusing screen 1 is in the optimum position thereof. In the event of a photo taking lens such as a wide-angle lens or the like that has an exit pupil position nearer to the focusing screen 1, for example, as indicated by a broken line, the direct incident light on the area 4C3 or 4C4 coming through the area 1C3 or 1C4 of the focusing screen 1 tends to be eclipsed. Therefore, when a lens having the exit pupil closer to the focusing screen 1, such as a wide-angle lens is mounted on the camera, the measured light quantity becomes inaccurate for the lower area of the image plane.
Meanwhile, there have been proposed many photometric devices of the kind arranged to divide the photographing field into a plurality of areas, to discretely measure light for each of these areas and to make an apposite exposure of the photographing image plane on the basis of the plurality of measured light values thus obtained. For example, the applicant of the present application has previously filed U.S. patent applications Ser. Nos. 513,153, 515,360, 563,462, 894,613, 009,995 and 043,935 for photometric devices of this kind.
The photometric devices of this kind measure the luminance of each of the divided area of the photographing field and perform a computing operation according to conditions for an apposite exposure selected from the plurality of photometric outputs thus obtained. Therefore, the device of this kind is required first of all to accurately measure the luminance of each area. However, in the event of a camera permitting use of interchangeable photo taking lenses, the degree of a light quantity drop which takes place in the peripheral part of the image plane greatly varies with the lens to be used. For example, expressing this on the basis of a full-open or maximum F-number, it varies up to a degree corresponding to two steps. Therefore, in the case of some lens, it is impossible to make accurate light measurement with the lens opened to the maximum aperture thereof and this eventually gives an erroneous result of the computing operation.
Further, a concept of correcting the above stated light quantity drop which takes place in the peripheral part of the image plane has been proposed in a U.S. Pat. No. 4,306,787 (Re 32376). The method employed in this concept, however, necessitates the camera to store therein some correction value information based on such lens information as F-number. This method presents no problem as long as the lens is selected from a group of anticipated interchangeable lenses. However, it presents a problem in cases where some special interchangeable lens is to be included in the interchangeable lens group. In other words, in designing the camera, the correction information can be completely stored at a ROM or the like for all the known intended interchangeable lens. However, it is impossible to cover an interchangeable lens newly designed after designing and marketing of the camera with known correction value information stored therein. Further, an interchangeable lens group generally includes about 30 lenses. Therefore, in cases where the correction information about every one of 30 lenses is to be stored by means of a ROM or the like, there arises a problem in terms of memory capacity.